1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for externally identifying a position of a ram inside a ram blowout preventer.
2. Discussion of the Background
During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has increased dramatically. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel.
The existing technologies for extracting the fossil fuel from offshore fields use a system 10 as shown in FIG. 1. More specifically, the system 10 includes a vessel 12 (e.g., oil rig) having a reel 14 that supplies power/communication cords 16 to a controller 18. The controller 18 is disposed undersea, close to or on the seabed 20. In this respect, it is noted that the elements shown in FIG. 1 are not drawn to scale and no dimensions should be inferred from FIG. 1.
FIG. 1 also shows a wellhead 22 of the subsea well and a drill line 24 that enters the subsea well. At the end of the drill line 24 there is a drill (not shown). Various mechanisms, also not shown, are employed to rotate the drill line 24, and implicitly the drill, to extend the subsea well.
However, during normal drilling operation, unexpected events may occur that could damage the well and/or the equipment used for drilling. One such event is the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such event is sometimes referred to as a “kick” or a “blowout” and may occur when formation pressure inside the well exceeds the pressure applied to it by the column of drilling fluid. This event is unforeseeable and if no measures are taken to prevent it, the well and/or the associated equipment may be damaged. Although the above discussion was directed to subsea oil exploration, the same is true for ground oil exploration.
Thus, a blowout preventer (BOP) might be installed on top of the well to seal the well in case that one of the above mentioned events is threatening the integrity of the well. The BOP is conventionally implemented as a valve to prevent the release of pressure either in the annular space between the casing and the drill pipe or in the open hole (i.e., hole with no drill pipe) during drilling or completion operations. Recently, a plurality of BOPS may be installed on top of the well for various reasons. FIG. 1 shows two BOPS 26 or 28 that are controlled by the controller 18.
A traditional BOP may be one to five meters high and may weight tens of thousands of kilograms. Various components of the BOP need to be replaced from time to time. An example of a BOP 26 is shown in FIG. 2. The BOP 26 shown in FIG. 2 has, among other things, two ram blocks 30 that are supported by respective piston rods 32 and a corresponding locking mechanism 33, which is configured to lock the rods 32 at desired positions. The two ram blocks 30 are configured to move inside a first chamber 34 (horizontal bore) along a direction parallel to a longitudinal axis X of the piston rods 32. The ram blocks 30 may severe the drill line 24 or other tools that cross a second chamber 36 (vertical wellbore) of the BOP 26. First and second chambers are substantially perpendicular to each other. However, after cutting the drill line 24 for a number of times (if a shear ram block is installed), the ram blocks 30 and/or their respective cutting edges need to be verified and sometimes reworked. For this reason, the BOP 26 of FIG. 2 is provided with a removable bonnet 38, for each ram block 30, which can be opened for providing access to the ram blocks. FIG. 2 shows the bonnet 38 having a hinge 40 that rotatably opens the bonnet 38.
FIG. 3 shows the BOP 26 having the bonnet 38 opened so as to expose the ram block 30. Thus, as can be seen from FIGS. 1 to 3, when the bonnet 38 is closed, the position of the ram block 30 cannot be ascertained. Further, when the BOP is operational, the ram block 30 may have a functional open position and a functional closed position. At least these two positions need to be known by the operator of the BOP.
These positions may be detected as disclosed, for example, in Young et al., Position Instrumented Blowout Preventer, U.S. Pat. No. 5,320,325 (herein Young 1), Young et al., Position Instrumented Blowout Preventer, U.S. Pat. No. 5,407,172 (herein Young 2), and Judge et al., RAM BOP Position Sensor, U.S. Patent Application Publication No. 2008/0196888, the entire contents of which are incorporated here by reference.
These documents disclose a magnetostrictive device for determining the position of the ram block 30 relative to the body of the BOP 26. These devices generate a magnetic field that moves with a piston connected to the ram block and disturbs another magnetic field generated by a wire enclosed by a tube. When this disturbance takes place, a magnetic disturbance propagates as an acoustic wave via the tube to a detector. The time necessary by the magnetic disturbance to propagate to the detector may be measured and used to determine the position of the piston relative to the body of the BOP.
Other techniques for measuring the position of the piston are known, for example, the use of a linear variable differential transformer (LVDT). LVDT is a type of electrical transformer used for measuring linear displacement. The transformer may have three solenoidal coils placed end-to-end around a tube. The centre coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube. An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.
As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change. The coils are connected in reverse series, so that the output voltage is the difference (hence “differential”) between the two secondary voltages. When the core is in its central position, equidistant between the two secondaries, equal but opposite voltages are induced in these two coils, so the output voltage is zero.
When the core is displaced in one direction, the voltage in one coil increases as the other decreases, causing the output voltage to increase from zero to a maximum. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to a maximum, but its phase is opposite to that of the primary. The magnitude of the output voltage is proportional to the distance moved by the core (up to its limit of travel), which is why the device is described as “linear.” The phase of the voltage indicates the direction of the displacement.
Because the sliding core does not touch the inside of the tube, it can move without friction, making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the LVDT to be completely sealed from its environment. LVDTs are commonly used for position feedback in servomechanisms, and for automated measurement in machine tools and many other industrial and scientific applications.
However, these devices require a continuous source of power for measuring and transmitting the signals corresponding to the position of the ram block. Thus, in case of failure to receive electrical power from the power source, e.g., communication lost with the power source, the well operator is left without any indication about the position of the ram block.
Alternatively, well control operators rely on flow readings of fluid flow through the ram BOP in order to determine ram functionality. For example, a well control operator may fully open a ram BOP, measure the fluid flow through the ram BOP, and compare the measured fluid flow to an expected fluid flow. The well control operator may also fully close a ram BOP and measure whether any fluid flows through the ram BOP. Based on these readings, the positions of the rams in between the open and closed positions may be extrapolated. However, these techniques introduce a certain amount of uncertainty because the expected flow of fluid through the ram BOP may not be accurate. For example, the composition of the fluids flowing through the BOP may change such that measurements taken may be misleading.
Therefore, it is desired to provide a novel BOP for which the position of the ram block can be ascertained by other means than those discussed above.